The vote in the autonomous Kurdish region of northern Iraq and some disputed areas is non-binding and will not lead automatically to independence, but is seen by the Kurds as a major step towards a long-cherished dream of statehood.

Voters flocked to the polls, eager to show off their ink-stained fingers after casting their ballots, and an overwhelming "Yes" outcome is expected.

Polling stations for the five-million electorate closed at 7:00 pm (GMT) after voting was extended for an hour. Results were expected within 24 hours, with an overwhelming "yes" vote not in doubt.

The vote took place peacefully and in a festive atmosphere, but signs of potential trouble mounted as the day progressed.

In New York, UN chief Antonio Guterres expressed concern about the "potentially destabilising effects" of the referendum.

Expressing respect for "the sovereignty, territorial integrity and unity of Iraq", he called for differences to be resolved through "structured dialogue and constructive compromise".

Lawmakers in Baghdad, which has declared the vote unconstitutional, demanded the government send troops to disputed areas where the referendum was taking place.

In Istanbul, President Recep Tayyip Erdogan warned that Turkey -- which fears the effects of the vote on its own sizeable Kurdish population -- would shut its border with Iraqi Kurdistan and threatened to block key exports.

And in Kirkuk, a disputed city where the vote controversially went ahead, security forces deployed on the streets after a curfew was imposed in parts of the city.

- 'Day of celebration' -

Soon after voting began in the Iraqi Kurdish capital Arbil, many men headed to polling stations dressed in traditional Kurdish dress of brown shirt and billowing trousers.

"I came very early to be the first to vote for a Kurdish state," said Diyar Abubakr, 33.

"It's a day of celebration today. That's why I've put on our traditional outfit, which I bought for the occasion."

One voter even brought a cow to slaughter before the start of the referendum.

Veteran Iraqi Kurd leader Massud Barzani, who initiated the vote, smiled as he cast his ballot early in the morning, wearing a traditional headscarf.

He pushed ahead with the referendum despite the opposition from Baghdad and Ankara, but also from Iran which has its own large Kurdish minority, and Western countries who fear the vote could hamper the fight against the Islamic State (IS) jihadist group in which cooperation between Baghdad and the Kurds has been key.

Polling stations were scattered across the three northern provinces of autonomous Iraqi Kurdistan -- Arbil, Sulaimaniyah and Dohuk -- as well as in disputed border zones such as the oil-rich province of Kirkuk.

A total of 12,072 polling stations were open for more than 5.3 million registered voters.

Asked about the risks of armed conflict, Abadi's spokesman Saad al-Hadithi told AFP: "If there are clashes in these zones, it will be the job of federal forces to apply the law."

Karim al-Nuri, a head of the Badr Brigade which forms part of the powerful Hashed al-Shaabi paramilitary units, suggested the group was ready to deploy to "Kirkuk and the disputed zones occupied by armed gangs, outlaws who do not respond to the army command."

- Curfew in Kirkuk -

Kirkuk, home to Arabs, Kurds and Turkmen, has been a particular point of concern and late on Monday police said a curfew was being imposed in the centre of the city and in Arab and Turkmen areas "to ensure security, monitor the situation and protect the citizens of Kirkuk".

A senior official in Kirkuk later said the situation was stable and that the curfew would be lifted within hours. "Tomorrow will be a normal work day in Kirkuk," he said.

Abadi on Sunday pledged to take all the "necessary measures" to protect the country's unity, as his government urged all countries to deal only with it on oil transactions.

The Iraqi Kurds export an average 600,000 barrels per day through a pipeline running through Turkey to Ceyhan on the Mediterranean.